The present invention relates to a method for regenerating tetraalkylammonium hydroxide to be used as a developer in fine fabrication of circuit pattern and the like by photolithography in the manufacturing processes of integrated circuits such as LSIs or ultra LSIs or liquid crystal display units.
Semiconductor integrated circuits such as ICs, LSIs, ultra LSIs, etc. are produced as follows: Photo resist is coated on a substrate such as silicon wafer, and a desired pattern is put on it by exposure using a stepper. Then, photolithography process such as developing, etching, etc. are repeated. Thin film transistor (TFT) for liquid crystal display unit is also manufactured by photolithography process on a glass substrate.
Photo resist is divided into a negative type resist, in which irradiated portion is insolubilized by exposure of circuit pattern, and a positive type resist, in which irradiated portion is solubilized.
As a typical positive type photo resist, novolak resin containing photosensitive substance such as o-diazonaphthoquinone is used. An example of a mixture of esterized o-diazonaphthoquinone sulfonic acid ester and m-cresol type novolak resin is given by the following chemical formula: ##STR1##
The o-diazonaphthoquinone sulfonyl group combined with novolak resin gives an effect to decrease solubility of novolak resin. When light such as ultraviolet ray is irradiated on o-diazonaphthoquinone, it is turned to ketene, and 3-indene-carboxylic acid is further generated under the presence of water: ##STR2##
As the result, o-diazonaphthoquinone not only ceases to give effect as a dissolution inhibitor for novolak resin, but the alkali solubility is increased because acid is generated. Therefore, when a positive type photo resist with an exposed circuit pattern is developed by alkali solution, the exposed portion is dissolved by alkali solution, and a positive image is formed.
As alkali developer for the positive type resist, alkali carbonate, aqueous ammonia, tetramethylammonium hydroxide, etc. are used. With the increase of the devices with higher integration such as LSI, there are strong demands on the decrease of impurities in chemical agents to be used in the manufacturing process. Above all, there is strict restriction on intermingling of metal ions in the semiconductor manufacturing process. As the result, the developers containing tetramethylammonium hydroxide, which is an alkali solution containing no metal ion, as main component is now widely used in the process of photolithography.
Aqueous solution of hydroxide of tetraalkylammonium such as tetramethylammonium hydroxide used as a developer for the positive type photo resist had been disposed in the past as waste solution. This waste solution contains organic substances such as novolak resin, which is a major component of photo resist, and hydroxide of tetraalkylammonium contains nitrogen, which is turned to eutrophic component in water and causes contamination and deterioration of water quality. Thus, this has been disposed through various treatment procedures. With the increase in the quantity of developer used for the manufacture, the quantity of waste solution to be disposed also increased, and there have been urgent demands on the utilization of waste solution and on the decrease of the waste solution.
The present inventors have proposed a method for obtaining high purity tetraalkylammonium hydroxide from a cathode chamber in the Japanese Patent Application No. 2-408052 by introducing the waste solution into an anode chamber of an electrolytic cell, which is divided by a perfluorinated cation exchange membrane. According to this method, organic substances of novolak resin type in the waste solution, which is introduced into an anode chamber of an electrolytic cell, do not adversely affect the cation exchange membrane or move into the cathode chamber. But, when electrolysis is continued, concentration of organic substances such as novolak resin in the anode chamber increases, and the anode solution with increased organic substance concentration must be disposed, and it is impossible to completely regenerate and utilize tetraalkylammonium hydroxide in the waste solution.
In order to remove novolak resin and other organic substances from waste solution, it was attempted to apply membrane separation technique such as reverse osmosis method, ultrafiltration, etc. However, aqueous solution of hydroxide of tetraalkylammonium contained in the waste solution is strongly alkaline, and it is impossible to find a separation membrane, which is endurable to long-term use, and organic substances such as novolak resin in the waste solution cannot be separated by the application of the membrane separation technique.
It is an object of time present invention to provide tetraalkylammonium hydroxide, which has lower content of organic substances.